The acceptance of extracorporeal oxygenation devices based on gas transfer thru a membrane will depend to a large extent on the availability of simpler, easier to use, safer and lower cost devices. Oxygenators using microporous hollow-fibers have come into wider use the last five years. They present more membrane surface to blood and gas phases for equivalent volume (i.e. lower priming volume) and do not require a supplemental secondary flow for adequate gas transfer. These devices do however, suffer from the real and potential problems associated with all membrane oxygenators made from microporous materials - possible blood damage due to microbubble intrusion, excessive water transport and seepage (flooding) and a lack of selectivity in exchanging gases (oxygen and carbon dioxide). This proposal suggest an improvement to the present state-of-the-art microporous based hollow fiber oxygenators that eliminates all the problems listed above. We propose to develop a composite permselective/microporous hollow fiber device of optium geometry and membrane characteristics. The optimized hollow fiber geometry will be combined with PAS materials which have been shown to have superior permeability to carbon dioxide and oxygen and high bio-compatability with blood. Membrane oxygenator, blood perfusion, membrane lung, extracorporeal carbon dioxide removal, respiratory assist, oxygenation, ECMO, Bio-compatibility. Bubble oxygenators still dominate the oxygenator market, however a number of perfusion teams are shifting to membrane units. Approximately 500 individuals each day undergo heart surgery that requires extracorporeal oxygenation. The use of ECMO in the management of neonatal respiratory failure is also increasing.